I’ve been getting this error in WordPress, so I wasn’t using the auto-update functionality, which is a real time-saver. My problem was that WordPress was trying to write to /tmp/ which is a no-no as my host (Segpub.net) uses Safe Mode. You can solve this issue by adding a couple of directives to your wp-config.php file, namely WP_TEMP_DIR. Adding these lines to wp-config.php fixed the problem for me: